The invention pertains to a process and apparatus for drying suspensions of particulate solid materials.
Many different types of apparatus are available for thermal drying of wet particulate materials, but most suffer from the disadvantage that the particles tend to agglomerate during the drying process and the particle size distribution of the dry product tends to be significantly coarser than that of the wet feed material. The problem is particularly evident when the particulate material is finely divided, for example when it has a particle size distribution such that more than about 60% by weight consists of particles having an equivalent spherical diameter smaller than 2 xcexcm, and when the particulate material is highly concentrated in the aqueous medium, for example when the solids content of the suspension is in excess of about 50% by weight. For this reason, many finely divided particulate materials are made available commercially in the form of concentrated aqueous suspensions, and it is not commercially attractive to produce them in the form of a dried powder.
For many end uses, and especially when the particulate material is to be dispersed in a non-aqueous or non-polar medium, such as, for example, when it is to be used as a filler or extender in an organic polymer composition such as a rubber, plastics or non-aqueous paint composition, a finely divided particulate material in dry powder form would be commercially and technologically advantageous.
Transporting a particulate material in aqueous suspension form has the obvious disadvantage that a significant quantity of water is associated with the particulate material, which adds to the cost of conveying the material.
According to a first aspect of the present invention, there is provided an apparatus for drying a suspension of a finely divided particulate solid material in an aqueous or non-aqueous medium to thereby produce a substantially dry product, comprising:
(a) a grinding chamber containing a particulate grinding medium, said grinding chamber having an inner wall and a perforated base through which a heated gas is introduced to provide an upflow of gas through the grinding chamber and through the grinding medium, said perforated base having a central imperforate area which causes the gas to pass preferentially through a region near the walls of the grinding chamber;
(b) means for introducing the suspension into the grinding chamber; and
(c) an impeller capable of rotation in the grinding chamber, said impeller being provided with impeller bars.
The gas to be introduced through the perforated base of the grinding chamber is advantageously compressed by means of a fan and may be heated indirectly by passage through one side of a heat exchanger, or directly by contact with electric heating elements, or by passage through a combustion chamber in which a gaseous, liquid or solid fuel is burned.
The perforated base of the grinding chamber preferably has a central imperforate area which causes the gas to pass preferentially through the region near the walls of the grinding chamber. A top plate having a central opening is preferably positioned in the grinding chamber at a height above the perforated base which is not greater than one half of the diameter of the grinding chamber. The top plate is provided to prevent a bed of particulate solid material in the grinding chamber from extending upwards to an undesirable degree near the walls of the grinding chamber.
Classifying means for separating the substantially dry product into fractions of different particle sizes is preferably provided downstream of the grinding chamber. The classifying means may be separate from the grinding chamber and connected thereto by suitable conduits or may be mounted above the grinding chamber and may form an integral part therewith. The classifying means may conveniently be of the type in which a cylindrical or frusto-conical cage, comprising peripheral, longitudinally extending blades or vanes, is rotated about its longitudinal axis in a current of a suspension of particles in a gas. Relatively fine particles in the suspension pass between the blades or vanes of the rotating cage, while relatively coarse particles are deflected by the blades or vanes in the radially outward direction of the cage.
Advantageously, means are provided to inject additional gas into the mixture of gas and particles between the grinding chamber and the classifying means or into the classifying means itself. This arrangement makes it possible to adjust the flow rate of gas through the grinding chamber independently of the flow rate of gas through the classifying means, with the result that conditions in the grinding chamber can be adjusted to give an optimum product which will enable the classifying means to operate with the greatest precision and efficiency. Advantageously, a suction fan is provided downstream of the classifying means to provide negative pressure which will draw gas and suspended particles at the desired rate through the classifying means.
Filter means are also preferably provided downstream of the grinding chamber, (and of the classifying means, if used) to separate solid particles from the suspending gas. These means may be, for example, one or more cyclones and/or a bag filter assembly.
According to a second aspect of the invention, a process is provided for drying a suspension of a finely divided particulate solid material in an aqueous or non-aqueous medium to thereby produce a substantially dry product, comprising the steps of:
(a) introducing the suspension into a grinding chamber containing a bed of particulate grinding medium;
(b) rotating an impeller provided with impeller bars present in the grinding chamber so as to agitate the grinding medium; and
(c) introducing a heated gas through a perforate base of the grinding chamber such that it passes through the bed of the grinding medium, said substantially dry product being entrained by the gas and conveyed out of the grinding chamber.
Preferably, in practicing the process of the invention, the impeller will rotate in the grinding chamber at a peripheral speed in the range of about 5 to 20 m.sxe2x88x921, especially 8 to 11 m.s.xe2x88x921. Advantageously, the impeller will have a diameter d such that a gap in the range of about 0.01 d to 0.05 d is present between the ends of the impeller bars and the inner wall of the grinding chamber.
The grinding medium preferably comprises particles of diameter within the range from 0.5 to 12.5 mm. More preferably, the grinding medium comprises particles of diameter within the range from 1.0 to 5.0 mm. The grinding medium may comprise balls, beads or pellets of any hard mineral, ceramic or metallic material. Alternatively, particles of natural sand of appropriate size have been found to be very effective.
The process of the invention is particularly advantageous when the particulate solid material has a particle size distribution such that at least 60% by weight consists of particles having an equivalent spherical diameter smaller than 2 xcexcm. The suspension of the solid material is preferably a concentrated aqueous suspension and the process is particularly advantageous when the suspension has a solids content in excess of about 50% by weight, based on the weight of the suspension.
The suspension may be fluid and of relatively low viscosity, in which case a dispersing agent would generally be used. The dispersing agent may be, for example, a water soluble condensed phosphate, a water soluble salt of a polysilicic acid or a polyelectrolyte, for example a water soluble salt of a poly(acrylic acid) or of a poly(methacrylic acid) having a number average molecular weight not greater than 20,000. The amount of the dispersing agent used would generally be in the range of from 0.1 to 2.0% by weight, based on the weight of the dry particulate solid material. The suspension may be introduced at a temperature within the range from about 4xc2x0 C. to about 100xc2x0 C. However, it is advantageous to pre-heat the suspension to a temperature within the range from 25xc2x0 C. to 100xc2x0 C. Alternatively, the suspension may be in the form of a cake such as is formed in high pressure filtration of a relatively dilute suspension of the particulate solid material, provided that the cake is non-sticky. If the suspension is of the fluid type, it may be introduced into the grinding chamber through a conduit provided with a suitable inlet nozzle. If the suspension is in the form of a cake, it may be introduced through suitable ducting, the grinding chamber end of which may be closed by a rotating valve to prevent gas from passing from the grinding chamber into the ducting.
The gas containing entrained particles leaving the grinding chamber is preferably passed through dry classifying means to remove any particles having diameters greater than the desired maximum particle size. Generally it is required that the substantially dry product is free of any particles having an equivalent spherical diameter greater than 10 xcexcm, and the dry product is preferably free of any particles having an equivalent spherical diameter greater than 5 xcexcm.
The gas passing through the perforated base of the grinding chamber preferably has a temperature such that the temperature of the gas and suspended particles leaving the grinding chamber is at least 70xc2x0 C. It has also been found that a classifying means will not provide efficient separation if the gas which carries the particles to be separated is bone dry. Generally, for most efficient separation, it is required that the gas entering the classifying means has a relative humidity of at least 15%, but the relative humidity of the gas must not be so high as to cause condensation in later stages of the process. Generally the relative humidity of the gas should not be greater than about 55%. The particulate material is preferably dried to a water content of not more than 1% by weight, and preferably to a water content of not more than 0.2% by weight.
The fine particulate material which is dried in the grinding chamber may, at the same time be coated with a surface treatment agent which is conveniently introduced into the grinding chamber in particulate solid form through a duct which is provided at its grinding chamber end with a rotary valve. The surface treatment agent may be, for example, a fatty acid having from 12 to 24 carbon atoms in its hydrocarbon chain. Alternatively a liquid surface treatment agent may be used, such as a substituted silane substituted with at least one group having hydrophobic properties and at least one group which is compatible with the surface of the particulate solid material to be treated. Most suitably the substituted silane is of the type which is represented by the general formula: 
where R1 is an aminoalkyl or mercaptoalkyl group, R2 is a hydroxy, hydroxyalkyl or alkoxy group, and each of R3 and R4, which may be the same or different, is a hydrogen atom or a hydroxy, alkyl, hydroxyalkyl or alkoxy group.